Control unit for a wheel brake of a motor vehicle

ABSTRACT

An actuating unit for a wheel brake of a motor vehicle has a hydraulic pump drivable with variable speed by an electric motor, as well as an element, adjusting the return flow of pressurized media to the pump, which is arranged between the delivery side and the intake side of the pump. Furthermore, a cylinder-piston unit is provided, having a piston with identical surfaces on both sides, as well as a cylinder having cylinder chambers arranged on both sides of the piston. In order to press a brake lining against a friction element connected to a vehicle wheel, the pump is connected on the intake side to the cylinder chamber on the brake-lining side, and on the delivery side to the cylinder chamber facing away from the brake lining. The actuating unit is assigned a control device by which an electric brake request signal is able to be processed, and the electric motor and the element are controllable.

FIELD OF THE INVENTION

The present invention relates to an actuating unit.

BACKGROUND INFORMATION

An actuating unit for a wheel brake of a motor vehicle is described inGerman Published Patent Application No. 195 27 936, where avariable-speed electric motor drives a gear wheel pump, which draws inpressurized media from a reservoir and delivers it to a cylinder-pistonunit whose pistons are able to press a brake lining against a frictionelement connected to a vehicle wheel. Provided on the delivery side ofthe pump is a throttle which is used to control a connection to thereservoir. The pump drive and the throttle are controlled by anelectronic brake control to effect pressure build-up, pressuremaintenance and pressure reduction in the cylinder-piston unit.

The known actuating unit is disadvantageous insofar as apressurized-media-conducting connection to the reservoir is presentwhenever the brakes are actuated. Thus, the actuating unit is not aclosed hydraulic system, which is a significant safety shortcoming.Moreover, relatively large dimensions are required for the reservoir inorder to provide adequate pressurized media-volume given a seriouslyworn brake lining.

SUMMARY OF THE INVENTION

In contrast, the actuating unit according to the present invention hasthe advantage of an increased operational reliability due to the factthat it is a closed hydraulic system. When a braking action occurs, thesame pressurized-media volumes are constantly exchanged between thecylinder chamber of the cylinder-piston unit on the brake-lining sideand the cylinder chamber facing away from the brake lining. In thismanner, the changed piston position is compensated for even when thebrake linings are worn. If the hydraulic system is completely sealed, itwill not be necessary to replenish the pressurized media. Thissimplifies servicing the actuating unit, of which a passenger car, forinstance, has four. In addition, the actuating unit may be installed inthe vehicle already filled with pressurized media.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a circuit diagram of an actuating unit for a wheelbrake of a motor vehicle.

DETAILED DESCRIPTION

A vehicle, not shown in the drawing, has an actuating unit 1 for a wheelbrake 2 at each of its wheels. Actuating unit 1, represented in the formof a circuit diagram, has a cylinder-piston unit 3 whose cylinder 4 ispenetrated in its entire length by a piston rod 5. A piston 6 mountedthereon is provided with identical surfaces on both sides. In cylinder4, piston 6 separates two cylinder chambers 7 and 8 from one another,which are filled with hydraulic pressurized media. Piston rod 5 ofcylinder-piston unit 3 acts on at least one brake lining 9, which isable to be pressed against a friction element 10 connected to a vehiclewheel—in the case of a disk brake, a brake disk—and lifted off frictionelement 10.

A hydraulic pump 14, which may be driven at variable speeds by anelectric motor 13, constitutes part of actuating unit 1. On its intakeside 15, the pump is connected by an intake line 16 in apressurized-media conductive manner to cylinder chamber 7 on the brakelining side, and on its pressure side 17 by a pressure line 18 tocylinder chamber 8 of cylinder-piston unit 3 facing away from the brakelining. Furthermore, a return line 19 is provided, which connectspressure line 18 to intake line 16 and in which a 2/2 directionalcontrol valve is located, which assumes its passthrough position byspring activation and may be brought into its blocking position or anyother intermediate position by electromagnetic means. In addition,intake side 15 of pump 14 is coupled to a pressurized-media make-up tank21 having a small volume, which is sealed off against atmosphericinfluences by an air- and moisture-proof membrane 22. Lastly, ameasuring transformer 23, hereinafter called a pressure sensor,measuring the pressure in cylinder chamber 8 facing away from the brakelining, is connected to pressure line 18.

Actuating unit 1 as well as the additional actuating units assigned tothe further vehicle wheels are controllable by the vehicle driver via abrake pedal 26, whose pedal travel is detectable via a measuringtransducer 27, hereinafter called a travel sensor. Furthermore, anelectronic control device 28 is provided, via which the signalrepresenting an electric brake request signal of travel sensor 27 isable to be processed. Control device 28 is able to control electricmotor 13 and directional control valve 20 of all actuating units 1 ofthe vehicle. In addition, the signals measured by pressure sensors 23 ofactuating units 1 are able to be processed in control device 28.Finally, slip-sensitive signals, which are detected at the vehiclewheels by wheel-speed sensors (not shown) are able to be processed incontrol device 28 for the purpose of slip control, for instance, for ananti-lock protection control, drive slippage protection control, or fordirectional stability control of the vehicle.

Actuating unit 1 functions as follows:

With the generation of a brake request signal, be it by actuating thebrake pedal or by the wheel slippage control of the vehicle, not furtherdescribed, control device 28 activates electric motor 13 of pump 14, anddirectional control valve 20 is switched to its blocking position. Pump14 draws in pressurized media from cylinder chamber 7 located on thebrake-lining side, and delivers it to cylinder chamber 8 ofcylinder-piston unit 3 facing away from the brake lining. In thedrawing, piston 6 with piston rod 5 is shifted to the left, and the atleast one brake lining 9 is pressed against friction element 10 as afunction of pump pressure. In doing so, the pressure increase speed maybe adjusted via the variable speed control of electric motor 13 as afunction of the brake request signal pattern. The pressure exerted incylinder chamber 8 is detected by pressure sensor 23 and its signal isprocessed in control device 28. When reaching the pressure in cylinderchamber 8 that is a function of the brake request signal, directionalcontrol valve 20 is brought into an intermediate setting, so that thepressurized media delivered by pump 14 is diverted through return line19 to intake-side 15 of the pump, while maintaining the brake pressure.In order to reduce, and possibly entirely dissipate the pressure incylinder chamber 8, the intermediate setting of directional controlvalve 20 is changed, or it is switched to its flow-through setting. Dueto the elasticity of wheel brake 2, piston 6 is reset, and thepressurized media diverted from cylinder chamber 8 facing away from thebrake lining is returned to cylinder chamber 7 facing the brake lining.

During the entire braking period, electric motor 13 is switched on andpump 14 constantly supplies pressurized media. Depending on thepressurized-media demand in cylinder chamber 8 of cylinder-piston unit 3facing away from the brake lining, the pressurized media return-flow tothe pump through return line 19 is adjusted by directional control valve20. In this context, the gradient of the pressure build-up and pressurereduction in cylinder chamber 8 may be controlled by directional controlvalve 20 in cooperation with pump 14. Furthermore, actuating unit 1allows control of the brake pressure for the purpose of controllingslippage when starting from rest and during braking, or for controllingdirectional stability and for braking independently of the driver.

Pressurized media make-up tank 21 is not really required for theoperation of actuating unit 1, since the described hydraulic system is aclosed system, and pressurized media is merely exchanged between the twocylinder chambers 7 and 8 upon braking. In practice, make-up tank 21 maybe used to replace pressurized media lost at sealing points ofcylinder-piston unit 3 or pump 14, or for compensating for heat-relatedvolume fluctuations of the pressurized media in the system.

Actuating unit 1 is a system autonomous for each vehicle wheel. In anadvantageous manner, each actuating unit of the vehicle is connected toa separate power supply, or groups of actuating units are connected toseparate power supplies. In case of malfunction of one energy supply,the remaining actuating units remain operable.

What is claimed is:
 1. An actuating unit for a wheel brake of a motorvehicle, comprising: an electric motor; a hydraulic pump drivable withvariable speed by the electric motor; an element located between adelivery side and an intake side of the hydraulic pump and for adjustinga pressurized-media return flow; a cylinder-piston unit connected to thehydraulic pump on the delivery side, by which at least one brake liningis able to be pressed against a friction element connected to onevehicle wheel and lifted off therefrom; and a control device by which anelectric brake request signal is able to be processed and by which theelectric motor and the element for adjusting the pressurized-mediareturn flow are controllable, wherein: the cylinder-piston unit includesa piston having identical surfaces on both sides and a cylinder havingcylinder chambers arranged on both sides of the piston, and the intakeside of the hydraulic pump is connected to the cylinder chamber on abrake-lining side.
 2. The actuating unit according to claim 1, wherein:the hydraulic pump is constantly driven when the wheel brake isactuated, and the element for adjusting the pressurized-media returnflow is a 2/2 directional control valve having a number of intermediatesettings.
 3. The actuating unit according to claim 2, furthercomprising: a measuring transducer for measuring a pressure andconnected on the delivery side of the hydraulic pump, wherein: a signalproduced by the measuring transducer is able to be processed in thecontrol device.
 4. The actuating unit according to claim 1, furthercomprising: a pressurized-media make-up tank to which the intake side ofthe hydraulic pump is connected.